The present invention relates to an automatic direction stabilization system for aircraft rolling on the ground, especially rolling out after touchdowns.
Aircraft rolling out after touchdown may be subject to swerving moments about the yaw axis resulting from diverse causes. Such swerving moments may be effective while the aircraft is on a straight run or while it makes a turn. Such possibly uncontrollable perturbations could be caused by cross winds or gusts, for example. Aircrafts employimg thrust reversal may also be subject to such perturbations because the thrust reversal may cause a strong turbulence. Aside from these aerodynamical moments it is possible that the condition of the runway may have a considerable effect on the directional stability of the rolling aircraft, for example, when the friction between the wheel and the runway is reduced due to so-called aqua planing or due to snow and ice covering the runway. The just described effects may also be caused by wet grass or mud when aircrafts are landing on dirt strips, auxiliary airfields or the like.
Electric control circuit means for preventing swerving or skidding motions of an aircraft are known and are used for aircraft equipped with wheel brakes. Such a system releases the brake pressure on the respective brake when there is a danger of blocking of the wheel or where there is a tendency that the differential r.p.m. between two wheels being braked increases beyond a permissible limit. Besides, methods are known to use the steering mechanisms necessary for taxiing on the ground or also controlling the aircraft during the roll out after touchdown to keep the aircraft rolling in the desired direction. This control may be accomplished either through a differential action on the left or right main brake or, in case the aircraft is equipped with means for steering the nose wheel or tail wheel, the pilot may take correctional steering actions with regard to such nose and/or tail wheel. Thus, U.S. Pat. No. 2,953,323 describes an example of a hydroelectric nose wheel steering system including foot pedals controlled by the pilot.
However, in aircrafts of all kinds it happens that after landing the perturbations described above lead to situations where neither the known anti-skid systems nor corrective actions by the pilot through wheel brakes, nose and/or tail wheel steering means or rudder can provide sufficient directional stability. As a result, swerving of the aircraft about the yaw axis, deviations from the runway and possibly heavy damage or even the total loss of an aircraft have occurred in the past.